Method and leadframe for packaging integrated circuits

ABSTRACT

A leadframe suitable for use in the packaging of at least two integrated circuit dice into a single integrated circuit package is described. The leadframe includes a plurality of leads. Each of a first set of the plurality of leads has a first side and a second side substantially opposite the first side of the lead. Additionally, each of the first and second sides of the first set of leads each include at least two solder pads. Each solder pad on a lead of the first set of leads is isolated from other solder pads on the same side of the lead with at least one recessed region adjacent the solder pad. In various embodiments, I/O pads from at least two dice are physically and electrically connected to the opposing sides of the leads.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of and claims priority to U.S. patentapplication Ser. No. 11/961,842, entitled “Method and Leadframe forPackaging Integrated Circuits,” filed Dec. 20, 2007, which is herebyincorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

The present invention relates generally to the packaging of integratedcircuits (ICs). More particularly, an IC package is described thatincludes two IC dice that share common leads.

BACKGROUND OF THE INVENTION

There are a number of conventional processes for packaging integratedcircuit (IC) dice. By way of example, many IC packages utilize ametallic lead frame that has been stamped or etched from a metal sheetto provide electrical interconnects to external devices. The die may beelectrically connected to the lead frame by means of bonding wires,solder bumps, or other suitable electrical connections. In general, thedie and portions of the lead frame are encapsulated with a moldingmaterial to protect the delicate electrical components on the activeside of the die while leaving selected portions of the lead frameexposed to facilitate electrical connection to external devices.

In some applications, it is desirable to leave the back surface(opposite the active surface) of the die exposed; that is, not toencapsulate the back surface of the die with molding material. By way ofexample, it may be desirable to leave the back surface of the dieexposed in order to increase heat dissipation out of the die. This isespecially relevant for packages used in power applications. Increasingheat dissipation out of an IC die generally results in greater deviceperformance and stability.

While existing arrangements and methods for packaging IC devices workwell, there are continuing efforts to improve the thermal performance ofIC devices.

SUMMARY OF THE INVENTION

In one aspect, an integrated circuit package is described that includestwo dice. The active surface of each die includes a plurality of I/Opads. The active surface of the first die is positioned adjacent firstsides of the leads of a leadframe such that I/O pads from the first dieare arranged adjacent corresponding solder pads on the first sides.Similarly, the active surface of the second die is positioned adjacentsecond sides of the leads opposite the first surfaces such that I/O padsfrom the second die are arranged adjacent corresponding solder pads onthe second sides. Each of a first set of selected leads includes atleast two solder pads on each of the first and second sides of the lead.Each solder pad on a selected lead is isolated from other solder pads onthe same side of the lead with at least one recessed region adjacent thesolder pad. A plurality of solder joints are arranged to physically andelectrically connect I/O pads from the first or second die to associatedadjacent solder pads on the leads. The solder from each solder jointthat contacts an associated solder pad surface on a selected lead isconfined to the solder pad surface by one or more adjacent recessedregions. In this way, a single leadframe can be utilized to package twodice, one on either side of the leads of the leadframe.

In another aspect, a method of packaging at least two integrated circuitdice into a single integrated circuit package utilizing a singleleadframe is described. A first die is positioned onto a first side of aleadframe such that I/O pads on the first die are positioned adjacentcorresponding solder pads on first sides of selected leads of theleadframe. Solder bumps positioned between the I/O pads on the first dieand the solder pads on the first sides of the leads are then reflowed toproduce a first plurality of solder joints that physically andelectrically connect the first die to the leads of the leadframe.Subsequently, the leadframe is positioned onto a set of spacers suchthat the first die is below the leadframe. The spacers are arranged tosupport the leadframe such that the die does not have to contact anyother surface. A second die is positioned onto a second side of theleadframe opposite the first side of the leadframe such that I/O pads onthe second die are positioned adjacent corresponding solder pads onsecond sides of the selected leads of the leadframe. Solder bumpspositioned between the I/O pads on the second die and the solder pads onthe second sides of the leads are then reflowed to produce a secondplurality of solder joints that physically and electrically connect thesecond die to the leads of the leadframe. The spacers support theleadframe such that during the second reflowing the first plurality ofsolder joints are not compressed by the weight of the leadframe and arethus able to retain their shape.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference should be made tothe following detailed description taken in conjunction with theaccompanying drawings, in which:

FIGS. 1A-D illustrate diagrammatic first cross-sectional side, secondcross-sectional side, cross-sectional top and bottom views,respectively, of an IC package in accordance with an embodiment of thepresent invention.

FIGS. 2A-B illustrate diagrammatic cross-sectional side andcross-sectional top views of the IC package of FIG. 1 mounted on aprinted circuit board in accordance with an embodiment of the presentinvention.

FIGS. 3A-B illustrate diagrammatic cross-sectional side and top views,respectively, of a three-die package in accordance with an embodiment ofthe present invention.

FIG. 4 is a flow chart illustrating a process of packaging integratedcircuit dice in accordance with an embodiment of the present invention.

FIG. 5 illustrates an arrangement in which dice are mounted to eachother on two opposite sides of a leadframe.

Like reference numerals refer to corresponding parts throughout thedrawings.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The present invention relates generally to the packaging of integratedcircuits (ICs). More particularly, an IC package is described thatincludes two IC dice that share common leads.

In the following description, numerous specific details are set forth toprovide a thorough understanding of the present invention. It will beapparent, however, to one skilled in the art that the present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process steps have not been described in detail inorder to avoid unnecessary obscuring of the present invention.

Various embodiments of the present invention will be described withreference to FIGS. 1-2. Aspects of the present invention provide an ICpackage that utilizes a leadframe in the packaging of at least two ICdice. The I/O pads on the active surfaces of the dice are physically andelectrically connected with associated leads of the leadframe withsolder joints. In some embodiments, the dice are packaged such that anexposed metallic layer deposited onto the back surface of at least oneof the dice remains uncovered by molding compound used to encapsulateother portions of the dice, leads and solder joints.

Referring initially to FIGS. 1A-1D, an IC package 100 is described. ICpackage 100 is particularly suitable for use in power applications.Generally, package 100 may be referred to as a flip-chip-on-lead (FCOL)type package. FIGS. 1A and 1B illustrate cross-sectional side views ofpackage 100 taken along lines B-B and C-C, respectively, shown in FIG.1C, which illustrates a cross-sectional top view of package 100 takenalong line A-A of FIG. 1A. IC package 100 includes a first bottom IC die102 and a second top IC die 104. In some embodiments, both of the dice102 and 104 may be configured for use in power applications.

The first bottom die 102 and second top die 104 have active surfaces 106and 108, respectively. The active surfaces 106 and 108 are arranged soas to face one another and each include a plurality of bond pads 110(although the die 102 would be hidden from view, the perimeters of thedie 102 and bond pads 110 associated with the die 102 are illustratedwith a dotted line in FIG. 1C). In some particular embodiments, theactive surfaces 106 and 108 are mirror images of one another. Morespecifically, if the active surface 108 of the second top die 104 ispositioned over and adjacent the active surface 106 of the first bottomdie 102, the bond pads 110 on the active surface 108 of the top die 104may align with corresponding similar functioning bond pads 110 on theactive surface 106 of the bottom die 102. The bond pads 110 may be theoriginal bond pads on the active surface of the die 102 or otherinput/output (I/O) pads that have been redistributed from the bond padsusing various redistribution techniques (hereinafter, bond pads will beused interchangeably with I/O pads). Additionally, in variousembodiments, underbump metallizations (UBMs) may be formed on the bondpads 110 of the dice 102 and 104 prior to solder bumping.

In various embodiments, the bottom IC die 102 includes a thin metalliclayer 112 deposited onto the back surface 114 of the die as bestillustrated in FIG. 1D, which illustrates the bottom surface of thepackage 100. The thin metallic layer 112 may be formed from any suitablemetal or metallic alloy. By way of example, the thin metallic layer 112may be an alloy of titanium, nickel and silver. The thin metallic layer112 may also be applied to the back surface 114 of the die 102 with anysuitable means including, for example, sputtering. The metallic layer112 may serve as a heat dissipation medium for transferring thermalenergy out of the die 102. In various embodiments, the back surface 114of the die 102 is intended to be soldered directly to a desiredsubstrate, such as a PCB, to provide for enhanced heat dissipation outof the die. Since solder does not generally adhere well to Si, themetallic layer may serve as an intermediary between the solder and theSi. In other embodiments, such as in analog applications, it isdesirable to electrically connect the back surface 114 of the die 102 toa PCB to allow control over the electrical potential of the back regionof the die. In some embodiments, a similar thin metallic layer 116 isalso deposited onto the back surface 118 of the top die 104.

Package 100 additionally includes a leadframe having a plurality ofleads 120. Each lead 120 may be configured as a power lead intended forcoupling to an external power or ground line. By way of example, powerleads 120 may be configured to carry at least approximately 1 Watt. Inother applications, each power lead 120 may be configured to carry muchhigher powers. Each lead 120 includes an inner lead finger portion 120a, a middle lead portion 120 b and an outer lead portion 120 c. Invarious embodiments, the leads 120 are arranged such that the inner leadfinger portions 120 a are arranged in interlaced adjacent rows over theactive surface 106 of the first bottom die 102. More specifically, theleads 120 may be arranged such that the middle portion 120 b and outerportion 120 c of each lead 120 is positioned on an opposite side of thedie 102 as the middle and outer lead portions of the leads 120immediately adjacent to the respective lead.

In the embodiment illustrated in FIGS. 1A-D, which illustrate a dualinline package (DIP) format, four leads 120 are configured such that thefour associated inner lead finger portions 120 a are arranged in fourinterlaced rows over the active surface 106 of the bottom die 102. Theouter portions 120 c of the corresponding leads 120 are arranged suchthat two of the outer portions 120 c of the leads 120 extend from eachof two opposite sides of the package 100.

Furthermore, a number of signal leads 122 may be provided in addition tothe power leads 120. For example, in the illustrated embodiment, twosignal leads 122 are provided. The illustrated leads are arranged onopposite sides of one end of the package 100. In other embodiments,package 100 may include only one signal lead 122, while in still otherembodiments, package 100 may include more than two signal leads 122.Leads 122 are generally intended for connection to signal or controllines and any suitable number of signal leads 122 may be present inpackage 100. The associated inner regions 122 a of the leads 122 may bepositioned in a single row over the active surface 106 of the bottom die102 as illustrated in FIG. 1C. The outer portions 122 c of the leads 122are arranged to extend from opposite sides of the package 100. Thedescribed arrangement forms a DIP 100 having five rows of leads over theactive surface 106 of the die 102 and six corresponding external outerlead portions 120 c and 122 c, three of which (two outer portions 120 cand one outer portion 122 c) extend from each of two opposite sides ofthe package 100.

Package 100 may also include a number of additional internal leads 124.Internal leads 124 may be used to connect associated bond pads 110 onthe active surfaces 106 and 108 of the two dice 102 and 104 with oneanother. In the illustrated embodiment, the internal leads 124 do notextend out of the package 100. The leads 124 enable the dice 102 and 104to communicate with one another. By way of example, in one embodiment,one of the dice 102 or 104 may serve as a master chip that regulates theoperations of the other die. In one alternate embodiment, bond pads 110on each of the respective dice 102 and 104 may be connected with oneanother with larger solder joints that span the gap between associatedbond pads 110 on each of the respective dice thereby eliminating theneed for the leads 124.

Each of the inner lead portions 120 a and 122 a and the internal leads124 includes at least one conductive solder pad 126. The inner leadportions 120 a and 122 a are arranged such that the solder pads 126 arepositioned adjacent corresponding bond pads 110 on the active surfaces106 and 108 of the dice 102 and 104, respectively. Each bond pad 110 isphysically and electrically connected to one of the associated leads120, 122 or 124 with a solder ball joint 128. In various embodiments,the outer portions 120 c and 122 c of the leads 120 and 122 additionallyinclude package contacts 130 on the bottom surfaces of the leads. Insome embodiments, the leads 120, 122 and 124 may be etched, half-etched,or otherwise thinned relative to the solder pads 126 and/or packagecontacts 130 as will be described in more detail below.

It many embodiments, a single lead 120 or 122 may be electrically andphysically connected with one or more bond pads 110 on each of the dice102 and 104, as illustrated in FIGS. 1A and B. This leadframearrangement especially facilitates the packaging of dice 102 and 104whose respective active surface 106 and 108 and associated bond pads 110are mirror images of one another. More specifically, a single lead 120or 122 may be used to bias bond pads 110 on both dice 102 and 104simultaneously.

In various embodiments, one or more leads 120 are each connected withmultiple I/O pads 110 on each of the active surfaces 106 and 108 of thedice 102 and 104. By way of example, a single inner lead finger 120 amay include multiple solder pads 126, each of which is to be physicallyand electrically bonded to one of multiple I/O pads 110 designated forconnection with power or ground lines, which typically carry highercurrent and power. The number of I/O pads 110 connected with each lead120 may vary widely. By way of example, anywhere from 1 to 8 I/O pads110 on each die 102 and 104 may be connected with corresponding solderpads 126 on a single lead 120. In some high power applications, an evengreater number of I/O pads may be connected with a single lead 120. Inthe embodiment illustrated in FIGS. 1A-D, the leads 120 that areintended for connection to higher current power or ground lines and areeach connected with three corresponding I/O pads 110 on each of the topand bottom dice 104 and 102. In contrast, the leads 122 are generallyintended for connection to signal or control lines and are eachconnected with a single I/O pad 110 on each of the top and bottom dice104 and 102 via a single solder joint 128.

In some embodiments, the leads may be etched to form recessed regions121 around the solder pads 126 on both sides of the leads 120 in orderto prevent the spread of solder between adjacent solder pads 126 andalong other surfaces of each lead. The recessed regions 121 essentiallyform a moat around each solder pad 126 that serves to isolate the solderpad from the rest of the associated lead surfaces. The recessed regions121 may be formed by any suitable means. By way of example, the recessedregions 121 may be formed by etching the top surface of the lead framepanel. The formation and use of recessed regions to isolate solder padsis described in more detail in U.S. patent application Ser. No.11/691,429, which is incorporated by reference herein.

Each recessed region 121 is recessed sufficiently from the surface ofthe solder pads of the associated leads to prevent flux and solder fromspreading to undesired surfaces of the lead 120. More particularly, therecessed regions 121 are preferably etched sufficiently deep such thatthe spread of flux or solder is limited to the solder pads 126 by thesurface tension of the flux or solder, respectively. By way of example,the recessed regions 121 are preferably recessed to a depth in the rangeof approximately 2 to 4 mils in typical lead frame designs althoughdeeper or shallower recessed regions may be provided. In one preferredembodiment the recessed regions on both sides of the leads arehalf-etched simultaneously thus saving valuable processing time. Theaforementioned recess depths work well for a variety of solder padgeometries and sizes.

It should be appreciated that the resulting “raised” solder pads limitthe spread of solder since (a) they tend to define the areas cleaned byflux, and (b) the surface tension of the solder tends to further helpprevent the solder from extending beyond the edges of the solder pads212.

In one embodiment, the recessed regions 121 are etched such that thesolder pads 126 are substantially circular. In an alternate embodiment,the solder pads 126 may be substantially oval, rectangular or square(with or without rounded corners). However, in many applications it ispreferable to have substantially circular solder pads rather thanrectangular solder pads or other solder pads having geometries withsharp corners. More particularly, sharp corners may have the effect ofcounteracting the forces of surface tension that confine the flux andsolder to the surfaces of the solder pads 126. Additionally, in someapplications it will be desirable to form solder pads 126 wider thanother portions of their associated leads 208.

The recessed regions 121 preferably extend to a sufficient length alongthe leads so that the flux may not bridge the recessed regions betweenthe solder pads 126 and the rest of the leads. Additionally, in someembodiments it may be desirable for the recessed regions 121 to extendto a greater length.

It should be appreciated that the solder pads 126 defined by therecessed regions 121 may also be advantageously used to control thestandoff height between the leads and an associated die. The standoffheight between the leadframe (e.g., solder pad 126) and the die (e.g.,I/O pad 110) is generally a function of the volume of solder in thesolder bump 128 as well as the surface area and geometry of theassociated UBM (or I/O pad 110) and solder pad 126. Therefore, bycontrolling the volume of solder as well as the surface areas andgeometries of the solder pad 126 and I/O pad 110, a desired standoffheight may be achieved. Furthermore, since the same process may beapplied to every solder joint, a uniform standoff height may be achievedacross the entire die.

In the embodiment illustrated in FIG. 1A, the dice 102 and 104 areoffset from one another. The offset results from the staggering of thesolder pads 126 on opposite sides of the leads. More particularly, therecessed regions 121 in FIG. 1A are half etched and as such, the solderpads 126 on both sides of the lead 120 may not overlie one another. Inother embodiments, the recessed regions 121 may be recessed to a depthof less than halfway through the lead thereby allowing the solder pads126 on both sides of the lead to directly overlie one another therebypermitting the dice to directly overlie one another.

As will be appreciated by those familiar with the art, power or groundlines generally carry higher current than other signal or control lines.The aforementioned arrangement allows the current through a single lead120 to be shared by multiple associated I/O pads 110 on each of the topand bottom dice 104 and 102, and their associated solder ball joints128. The amount of current carried by each solder joint 128 is limitedin part by the size of the solder joint (e.g., the diameter of thesolder joint). The diameter of the solder joint 128 is, in turn,generally limited by the size of the corresponding I/O pad 110, which isin turn limited by the available real estate on the active surfaces 106and 108 of the dice 102 and 104. More particularly, for a given diefootprint, the layout (distribution), size and shape of the I/O pads 110is limited by the regions on the active surfaces of the dice availablefor bonding and the total area of the active surfaces of the dice aswell as proximity constraints placed on the I/O pads.

Those familiar with the art will appreciate that the current carryingand heat dissipation capabilities of solder ball joints far exceed thoseof bonding wires. Generally, as the number and diameter of the solderball joints 128 increase, the current carrying and heat dissipationcapabilities increase. Additionally, as the diameters of the solder balljoints 128 increase, the resistance through the solder ball jointsdecreases. As a result of their larger diameters and the relativelyshorter distance traveled through a solder ball joint as compared to atypical bonding wire, the electrical resistance through solder balljoints is far below that of typical bonding wires. By way of example, atypical solder ball joint may have a resistance of approximately 0.5 mΩwhile a corresponding bonding wire used in a similar application mayhave a resistance in the range of approximately 60 to 100 mΩ.

It will be appreciated by those skilled in the art that, although aspecific lead frame arrangement has been described and illustrated,embodiments of the present invention may utilize an extremely widevariety of other leadframe configurations as well. Additionally,although described with references to top and bottom dice and varioussurfaces, it should be appreciated that this context is intended solelyfor use in describing the structure and may not coincide with the finalorientation of the package after subsequent attachment to a PCB or othersuitable substrate.

In the illustrated embodiment, portions of the dice 102 and 104 andleads 120, 122 and 124 are encapsulated with a molding material orcompound 132. The molding compound is generally a non-conductive plasticor resin having a low coefficient of thermal expansion. Package 100 maybe encapsulated in such a way as to prevent molding material 132 fromcovering or intruding over the metallic layer 112 on the back surface114 of the bottom die 102. Package 100 may also be encapsulated suchthat molding material is prevented from covering or intruding over ametallic layer 116 on the back surface 118 of the top die 104. Themolding material does encapsulate other portions of the dice 102 and104, the solder joints 128, leads 124 and generally at least the innerportions 120 a and 122 a and middle portions 120 b and 122 b of theleads 120 and 122, respectively. In the embodiment illustrated in FIGS.1A-1D, the outer portions of the leads 120 and 122 extend from the sidesof the encapsulated package 100 and are bent into a characteristicgull-wing formation to facilitate electrical connection with a printedcircuit board (PCB) or other suitable substrate. Additionally, thepackage contacts 130 on the bottom surfaces of the leads 120 and 122 maybe configured so as to be coplanar with the bottom or back surface ofthe metallic layer 112.

In the embodiment illustrated in FIGS. 2A and B, the package contacts130 on the bottom surfaces of the leads 120 and 122 of package 100 arephysically and electrically connected with corresponding contacts 234 ona PCB 236 via solder joints 228. In various embodiments, the metalliclayer 112 is also physically and electrically connected to an associatedcontact surface 238 on the PCB 236. Additionally, in some embodiments, aheat sink 240 may be soldered to the metallic layer 116 on the top die104. In the illustrated embodiment, the heat sink 240 is also solderedto corresponding contact surfaces 242 on the PCB 236. Although aspecific heat sink 240 is illustrated, it will be appreciated that anysuitable heat sink may be incorporated.

The described arrangement provides multiple efficient and directmechanisms for dissipating heat out of the package 100. Moreparticularly, by soldering or otherwise connecting the metallic layer112 on the back surface 114 of the bottom die 102 to the PCB 236, adirect thermally conductive path is created between the die 102 and thePCB 236. Additionally, the heat sink 240 provides an efficient means fortransferring heat out of the top of the package 100. Furthermore, asalready described, the solder joints 128 also provide an efficientthermal path for dissipating thermal energy out of the package 100 viathe leads 120 to the contacts 234 on the PCB 236. Thus, embodiments ofthe present invention provide three efficient means of dissipating heatout of the package 100.

Furthermore, the described arrangement of the dice 102 and 104 and leads120, 122 and 124 enables the production of a package 100 having doublethe effective silicon density and hence potentially double theperformance while maintaining a conventional package size. Conversely,one could retain a desired silicon density and performance while halvingthe footprint of the package. More specifically, by advantageouslyutilizing the volume in the top half of the package to incorporate asecond die, the number of transistors for a given package footprint maybe doubled. Moreover, by soldering the exposed metallic layer on theback surface of the bottom die to a PCB and/or soldering a heat sink toan exposed metallic layer on the back surface of the top die, thethermal performance of the package is increased sufficiently to enablethe full utilization of both dice. Furthermore, in embodiments in whichmirror image dice are used, no additional leads are required as eachlead may be connected with corresponding I/O pads on both dice.

Although the embodiments described thus far have focused on thepackaging of two IC dice, it will be appreciated that, in otherembodiments, more than two dice may be packaged into a single IC packageas well. By way of example, FIGS. 3A and 3B illustrate an alternativeembodiment of a package 100 which includes two top dice 104 and 304. Inthe embodiment illustrated in FIG. 3B, the perimeter of the bottom die102 is illustrated with a dashed line while the perimeters of the topdice 104 and 304 are each illustrated with dotted lines. In someembodiments, one or both of the top dice 104 and 304 may be utilized asa control chip for regulating operations in the bottom die 102. In theseembodiments, bond pads 110 on one or both of the dice 104 and 304 may beconnected with bond pads on the bottom die 102 via internal leads 124.Depending upon the application, the top dice 104 and 304 may or may notinclude exposed metallic layers on their respective back surfaces. Insome embodiments, the top dice 104 and 304 may also be electricallyconnected with one another to permit electrical communicationtherebetween. As will be appreciated by those of skill in the art, thedescribed arrangement facilitates the production of system-in-package(SIP) or multi-chip module (MCM) package designs. Furthermore, althoughonly a single embodiment of a three-chip (die) package is illustrated,it will be appreciated that both the number and arrangement of the leadsand dice may vary widely.

With reference to FIGS. 4 and 5 a process 400 of packaging at least twointegrated circuit dice with a single leadframe device area will bedescribed. A leadframe such as those described above is provided at 402.At 404 a first die is positioned onto a first side of the leadframe. Itwill be appreciated that in various embodiments the leadframe is asingle leadframe device area of a larger leadframe panel having amultitude of leadframe device areas each of which is suitable for use inpackaging IC dice. By way of example, the leadframe panel may be in theform of a strip with side rails and other supporting structuressupporting the device areas of the leadframe panel. In theseembodiments, dice may be positioned on each device area of the leadframepanel. Additionally, in some embodiments, multiple dice may bepositioned within a single device area (such as in the embodimentillustrated in FIGS. 3A and 3B described above).

In various embodiments, one or both of the dice and/or leads of theleadframe panel include solder bumps deposited thereon. At 406, thesolder bumps are reflowed to form solder joints that physically andelectrically connect the solder pad surfaces on the leads and I/O padson the dice.

At 408 the populated leadframe or leadframe panel is flipped andsubsequently positioned on a set of spacers at 410. FIG. 5 illustrates asingle leadframe device area 500 having a first die 502 physically andelectrically connected to leads 504 on a first side of the leadframe viaa first set of solder joints 506. In the illustrated embodiment, siderails 509 of the leadframe panel are each positioned onto a spacer 508to support the leadframe such that the dice 502 do not have to rest onany other surface.

At 412 a second die (or set of dice) 510 is positioned onto the secondside of the leadframe such that I/O pads on the second die arepositioned over the same leads 504 used to connect the first die. Thepopulated leadframe 500 is then reflowed again at 414 to produce solderjoints 512 that physically and electrically connect the bottom die (ordice) 510 to the second sides of the same leads 504. It should be notedthat since the spacers 508 support the leadframe 500 during the secondreflow the weight of the leadframe, dice and solder does not compressthe first set of solder joints 506 which are also melted during thesecond reflow. Additionally, it has been observed that the cohesion ofthe solder in the solder joints 506 is sufficient to support the hangingdice 502 during the second reflow. Subsequently, the populated leadframepanel is then encapsulated at 416 and singulated to produce individualIC packages (if necessary) at 418.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that the specificdetails are not required in order to practice the invention. Thus, theforegoing descriptions of specific embodiments of the present inventionare presented for purposes of illustration and description. They are notintended to be exhaustive or to limit the invention to the precise formsdisclosed. It will be apparent to one of ordinary skill in the art thatmany modifications and variations are possible in view of the aboveteachings.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

1. A method of packaging at least two integrated circuit dice into asingle integrated circuit package utilizing a single leadframe, themethod comprising: positioning a first die onto a first side of aleadframe such that I/O pads on the first die are positioned adjacentcorresponding solder pads on first sides of selected leads of theleadframe, the first die including an active surface with the I/O padsand an opposing back surface; reflowing solder bumps positioned betweenthe I/O pads on the first die and the solder pads on the first sides ofthe leads to produce a first plurality of solder joints that physicallyand electrically connect the first die to the leads of the leadframe;positioning the leadframe onto a set of spacers such that the first dieis below the leadframe, the spacers being arranged to physically supportthe leadframe; positioning a second die onto a second side of theleadframe opposite the first side of the leadframe such that I/O pads onthe second die are positioned adjacent corresponding solder pads onsecond sides of the selected leads of the leadframe; and reflowingsolder bumps positioned between the I/O pads on the second die and thesolder pads on the second sides of the leads to produce a secondplurality of solder joints that physically and electrically connect thesecond die to the leads of the leadframe, whereby the spacers supportthe leadframe such that during the second reflowing the first pluralityof solder joints are not substantially compressed by the weight of theleadframe.
 2. A method as recited in claim 1, wherein the firstplurality of solder joints are melted during the second reflowing andwherein the cohesion of the solder in the first plurality of solderjoints is sufficient to support the weight of the first die.
 3. A methodas recited in claim 1, wherein selected I/O pads on the first die andselected I/O pads from the second die are connected to the same leads.4. A method as recited in claim 1, wherein multiple I/O pads on thefirst die are connected with a single selected lead and wherein multipleI/O pads on the second die are connected with the same single selectedlead.
 5. A method as recited in claim 1, wherein after the positioningof the leadframe on the set of spacers, the first die is elevated suchthat the back surface of the first die is not in contact with any othersurface.
 6. A method as recited in claim 1, wherein the leadframe is asingle leadframe device area of a larger leadframe panel that includes aplurality of device areas, each device area being arranged to supporttwo dice on two opposing surfaces of the device area, respectively.
 7. Amethod as recited in claim 1, further comprising encapsulating theleadframe panel and singulating the leadframe panel to produce aplurality of integrated circuit packages each including at least twodice.
 8. A method as recited in claim 1, wherein: the back surface ofthe first die faces in a first direction during the first reflowingprocess; and the method further comprises flipping the leadframe afterthe first reflowing and before the second reflowing such that the backsurface of the first die faces in a second direction, wherein the seconddirection is substantially different from the first direction that thefirst die faced during the first reflowing.
 9. A method as recited inclaim 1, wherein: the set of spacers extends out from a base structure;and the positioning of the leadframe onto the set of spacers holds thefirst die over and away from the base structure, such that the backsurface of the first die faces the base structure and is not in directcontact with the base structure.
 10. A method as recited in claim 1,wherein after the positioning of the leadframe onto the set of spacers,the first die is carried by the solder bumps that connect the first dieto the leadframe and is not directly physically supported by any otherstructure that underlies the back surface of the first die.
 11. A methodas recited in claim 1, wherein during the second reflowing, the firstplurality of solder bumps carries the weight of the first die.
 12. Amethod of packaging at least two integrated circuit dice into a singleintegrated circuit package utilizing a single leadframe, the methodcomprising: positioning a first die onto a first side of a leadframesuch that I/O pads on the first die are positioned adjacentcorresponding solder pads on first sides of selected leads of theleadframe, the first die including an active surface with the I/O padsand an opposing back surface, wherein the first die is positioned abovethe leadframe; reflowing solder bumps positioned between the I/O pads onthe first die and the solder pads on the first sides of the leads toproduce a first plurality of solder joints that physically andelectrically connect the first die to the leads of the leadframe;flipping the leadframe such that the first die is positioned below theleadframe; positioning the leadframe onto a set of spacers, wherein theset of spacers physically support the leadframe and are not in directcontact with the first die, the spacers helping to suspend the first dieover a bottom surface such that there is a gap between the back surfaceof the first die and the bottom surface, wherein the back surface of thefirst die is prevented from pressing directly against the bottomsurface; positioning a second die onto a second side of the leadframeopposite the first side of the leadframe such that I/O pads on thesecond die are positioned adjacent corresponding solder pads on secondsides of the selected leads of the leadframe; and reflowing solder bumpspositioned between the I/O pads on the second die and the solder pads onthe second sides of the leads to produce a second plurality of solderjoints that physically and electrically connect the second die to theleads of the leadframe, whereby the spacers support the leadframe suchthat during the second reflowing the first plurality of solder jointsare not substantially compressed by the weight of the leadframe andwherein the first plurality of solder joints maintain sufficientstructural integrity during the second reflowing such that the first dieis carried by the first plurality of solder joints during the secondreflowing without receiving additional support from any supportstructure that underlies and is in contact with the back surface of thefirst die.